The present invention relates to an optical fiber cable structure and, more particularly, to a cable structure incorporating a gel material to reduce microbending losses.
In the manufacture of optical fiber cables, one or more optical fibers are typically surrounded by sheathing and strength members, as required, to enable manufacture, installation and operation of the cable while preventing damage to the optical fibers. For example, steps are typically taken to prevent buckling of the optical fibers due to compressive longitudinal forces during manufacture or subsequently during the lifetime of the cable, in order to prevent significant microbending losses from occurring. Microbending losses increase the attenuation of the optical radiation in the fibers, limiting the length of transmission in a fiber; see, for example, the article entitled "Microbending Loss in Optical Fibers" by Gardner, appearing in the Bell System Technical Journal, Vol. 54, No. 2, pages 457-465, February 1975. One type of cable construction is shown in U.S. Pat. No. 4,241,979, where microbending losses are substantially reduced or prevented by decoupling the optical fibers from the surrounding cable. That is, the optical fibers, typically formed into ribbons, are placed inside an inner sheath with sufficient space between the ribbons and the sheath to prevent longitudinal forces on the sheath from substantially acting on the fibers. In addition, the ribbons are given a slight twist to reduce bending losses.
When optical fiber cables are deployed in undersea systems, the cables must be protected to prevent the ingress of water. Suitable waterblocking materials in use must yield under strains experienced when the cable is made or handled. Otherwise, movement of the optical fibers within the cable would be prevented and the fibers would buckle because they contact, with a relative small periodicity, a surface of the unyielding filling material. The smaller the periodicity of the fibers when contacting such an unyielding surface, the greater the potential for microbending loss.
Typically, microbending loss in undersea optical fiber cables is more difficult to control at long wavelengths than at short wavelengths. Thus, the requirements on the mechanical properties of a fiber cable filling material are typically substantially more severe for the cable that is to be used at 1.55 .mu.m, for example, than they are if the cable is to be used at shorter operating wavelengths of 1.3 .mu.m, for example.
In one exemplary fiber cable design, disclosed in U.S. Pat. No. 4,952,012, a thinwall alloy tube is used to encase a plurality of optical fibers, and a water resistant gel is inserted through the tube and is used to fill any interstices between the individual fibers. While this design produces little or no added transmission loss due to fiber bending, it provides less than full support to the fibers and allows some undesirable fiber movement when the cable is held in tension or relaxed after tensioning.